Nike's stuff is proprietary, the iPhone would need an external receiver.
I wonder if it would be at all possible that Nike makes a different transmitter that works with the iPhone's bluetooth. I don't see any reason why they couldn't. I guess the only question would be wether or not the market was large enough, but I don't see how it wouldn't be. I'd guess that the number of iPhone owners is pretty close to the number of nano owners when they originally released Nike+.